1. Field of the Invention
The present invention relates to control of a display device and, more particularly, to brightness control and power control of a plasma display panel (PDP).
2. Description of the Related Art
Accompanying the widespread use of flat displays such as LCD, EL, PDP, etc. and to further improve display quality and brightness, in recent years there has been an increasing demand for a brightness controller which makes it possible to freely adjust the brightness of display in the use thereof so as to meet a variety of environmental conditions, covering a range from a lowest level of almost a dark state to a highest brightness level, while maintaining stability in the display.
A variety of brightness controllers have heretofore been provided for the display devices. In AC-type plasma display devices, the data to be displayed is written in write/erase-scan for every line and, then, the display is maintained by sustain discharges. The brightness of display varies in proportion to the number of the sustain discharges in one V.sub.sync time interval which defines a vertical scanning period of a screen, and, hence, can be changed by changing the number of the sustain discharges.
The brightness is controlled in a digital manner (stepwisely), as mentioned above. In practice, however, the brightness is adjusted by a user in an analog manner (continuously) by using a variable resistor. To stably control the brightness, therefore, the analog value must be stably converted into a digital value.
In a conventional brightness controller, periodical BC (brightness control) pulses whose duration is proportional to a set value of the variable resistor are generated in a BC pulse generator. The BC pulses are latched by a signal H.sub.sync which defines a horizontal scanning period, in a latch circuit, to make a BC signal that is in synchronism with the signal H.sub.sync. During horizontal scanning periods in which the BC signal is at high level, drive signals that can cause the sustain discharges are applied to the PDP, and during horizontal scanning periods in which the BC signal is at low level, drive signals that cannot cause the sustain discharges are applied to the PDP.
In the above conventional brightness controller, the trailing edges of the BC pulses are occasionally very close to latch timing of a latch circuit, because the duration of the BC pulses can continuously vary according to the set value of the variable resistor. If the trailing edge of the BC pulse becomes very close to the latch timing of the latch circuit, the output of the latch circuit becomes unstable, making the brightness of the PDP unstable.
In general, when the whole screen is bright, this fluctuation such that the display is slightly brighter, or is slightly darker, than the mean brightness, and cannot be perceived by the human veiwer. When the same phenomenon takes place where the whole screen is dark, however, the fluctuation becomes much more larger than when the screen is bright and will be perceived by the human veiwer, the change appearing as flickering (flickering on the display screen).
Meanwhile, in the plasma display, the electric power is consumed in different amounts depending upon the rate at which display cells that are turned on (hereinafter referred to as a display rate), and the consumption of electric power increases with an increase in the display rate. Therefore, even when the brightness level is adjusted in the aforementioned manner, the consumption of electric power occasionally exceeds a permitted level. In order to prevent this situation, APC (automatic power control) is introduced to forcibly lower the brightness level so that the consumption of electric power is suppressed to be below the permitted level.
In a conventional APC function, the consumption of electric power is detected by detecting a mean current flowing through a high voltage power source for driving a PDP device. The detected current value is compared with a reference value, and periodical APC pulses, whose duration varies according to the comparison results, are generated in an APC pulse generator. The APC pulses are latched by the signal H.sub.sync and ANDed with the BC signal to make the BC signal narrow. During horizontal scanning periods in which the narrow BC signal is at high level, drive signals that cause the sustain discharges are applied to the PDP, and during horizontal scanning periods in which the narrow BC signal is at low level, drive signals that cannot cause the sustain discharges are applied to the PDP.
Since the duration of the APC pulses also continuously varies in response to the detected current value, the same problem as with the BC pulses arises when the trailing edge of the APC pulse becomes very close to the latch timing of the latch circuit.